Recently, according to the consideration to global environment, polylactic acid, which is an ecological plant-derived general-purpose expandable resin, has attracted attention as a carbon-neutral material which takes the place of conventional general-purpose resins produced by petroleum resources. A molded article of expanded polylactic acid resin beads can be obtained as an in-mold molded article having a desired shape without being subject to any shape-related restrictions, like conventional molded articles of expanded polystyrene beads and molded articles of expanded polyolefin beads. In addition, in accordance with the intended object thereof, for example, for lightweightness, buffer performance, heat insulation performance and the like, design of properties is easily performed, and therefore the mold article is a promising material.
From the viewpoint of the heat resistance and the mechanical properties of polylactic acid resin molded articles, a crystalline polylactic acid resin is mainly used as the base resin of expanded polylactic acid resin beads for in-mold molding. However, expanded beads of a crystalline polylactic acid resin are difficult to fuse together during in-mold molding, as compared with expanded polystyrene resin beads and the like, and thus have a problem in point of fusibility thereof.
As a technique of improving the fusibility of expanded beads of a crystalline polylactic acid resin, for example, PTL 1 discloses expanded polylactic acid resin beads, in which each bead is composed of a core layer formed of a polylactic acid resin and an outer layer positioned on the surface side relative to the core layer, and the outer layer is formed of a polylactic acid resin. According to this technique, not only the whole expanded beads have a specific endothermic calorific value, but also the endothermic calorific value in the surface layer part of the expanded bead and the endothermic calorific value in the center part of the expanded bead satisfy a specific relationship, and therefore, the fusibility of the expanded beads can be enhanced.
On the other hand, depending on the use thereof, for example, in the case where a molded article of expanded polylactic acid resin beads is used as a core material or the like in a molded article of a FRP (fiber reinforced plastic) composite, the molded article of expanded beads may be exposed to a solvent such as styrene or the like contained in a thermosetting resin composition. At that time, when the solvent resistance of the molded article of expanded beads is insufficient, the expanded beads would separate owing to solvent penetration into the article, whereby the shape of the molded article of expanded beads could not be maintained, and therefore, a molded article of expanded beads having solvent resistance is desired in some cases. However, PTL 1 has no disclosure relating to a technique of imparting solvent resistance to multilayer expanded polylactic acid resin beads, especially in the case where an amorphous polylactic acid resin is used as the outer layer of the expanded beads, and there still remains room to study in the technique.
PTL 1: W02011/145391